I have changed the menu type from "Item Listings/Categories" to the new type "Latest items from ...". But this leads to the following PHP error when I select the menu:
"Warning: Invalid argument supplied for foreach() in D:\xampp\htdocs\joomla_dev\components\com_k2\views\latest\view.html.php on line 120". I also got a different error but I can't remember the exact text.
When I create a new menu of this type everything works fine.

You have to choose at least one user/category